The present invention relates to a method for manufacturing microcapsules having a wall film formed of a synthetic polymer system, and, in particular, to a method with which microcapsules having a polyurethane urea resin wall film can be manufactured with a high efficiency while stabilizing the quality of the microcapsules.
Conventional methods for manufacturing microcapsules, in which an oil solution containing therein a capsule core material and a water solution is emulsified and dispersed, a synthetic polymer system wall film is formed on a solution-drop interface, and the hydrophobic liquid surface of the microcapsule is covered with the synthetic polymer system wall film, include:
(1) A method for manufacturing microcapsules in which the emulsifying reaction is started in an area which is rapidly stirred and mixed thereby to produce an emulsified solution. Next, a reaction thereby produced is moved to a multi-stage cascade in a receptacle, and the temperature of the product is increased stagewise while being maintained constant within each of the stages. (See U.S. Pat. No. 4,761,255.) PA1 (2) A method for manufacturing microcapsules in which an emulsified and dispersed solution is passed through a pipe-shaped reactor successively and allowed to react at a temperature of 40.degree. C.-95.degree. C. (See U.S. Pat. No. 4,454,083.) PA1 (3) A method for manufacturing microcapsules in which a polyisocyanate insoluble in the oil solution is mixed in an inert organic phase, the thus-produced mixture is successively emulsified in a water phase, diamine or polyamine is added to the resultant emulsified solution, and the emulsified solution with diamine or polyamine is added to an interface with the polyisocyanate. (See Unexamined Japanese Patent Publication No. Sho-61-21728.) PA1 (4) A method for manufacturing microcapsules in which, when a hydrophobic oil solution film is covered with a synthetic polymer system wall film, water vapor is directly injected into a dispersion solution for heating processing. (See Unexamined Japanese Patent Publication No. Hei-2-139030.)
However, in method (1) disclosed in U.S. Pat. No. 4,761,255 and in method (3) disclosed in Unexamined Japanese Patent Publication No. Sho-61-21728, the wall-film-forming reaction is effected in a stirring tank having a vapor-liquid interface where capsule particles have a tendency to adhere to the vapor-liquid interface of the stirring tank. If the capsule particles adhere to the interface, then the heat transfer efficiency of the tank is reduced, with the result that the wall-film-forming reaction is insufficient. This results in a reduced quality of the product, and also in mixing of impurities when the tank is switched for production of other types of products. Therefore, it is necessary to clean off such adhered particles periodically, so that the production of microcapsules is thus unavoidably interrupted, which results in reduced production as well as in increased costs of production.
As a countermeasure against these problems, in method (2) disclosed in U.S. Pat. No. 4,454,083, there is proposed a method for passing successively an emulsified solution through a pipe-shaped reactor to effect a wall-film-forming reaction. However, in this method, because the stirring in the pipe-shaped reactor is not sufficient, there is produced a cohered capsule. Otherwise, because the wall-film-forming reaction takes a given holding time, there is required a large-scale apparatus. Also, if the capsule particles adhere to the pipe-shaped reactor, it is difficult to clean the reactor.
Moreover, in method (4) disclosed in Unexamined Japanese Patent Publication No. Hei-2-139030, there is proposed a method for injecting water vapor directly into an emulsified solution for heating processing. However, in this method, since the water vapor is injected into the solution before a capsule wall film is formed completely, the emulsion may be destroyed or split again, which produces a wide distribution of particle diameter, and adhesions may be produced in the vapor-liquid interface.